Integrated circuit fabrication sequences typically have a plurality of pattern delineation steps. In such steps, the substrate is coated with a radiation sensitive material, commonly termed a resist, and selected portions of the resist are exposed to radiation which induces reactions within the resist that yield to differential removal rates between the exposed and unexposed portions of the resist. After portions of the resist have been removed, the now exposed portions of the underlying substrate are removed by etching. Various materials, such as an organic antireflection coating, silicon dioxide, silicide, polysilicon, and metal may be part of the substrate underneath the resist. If present, the antireflection coating may also have a planarizing function.
The pattern is typically transferred from a mask to the resist by exposing selected portions of the resist to radiation which passes through portions of the mask. The transfer must be accurate, but unfortunately, it is a function of many parameters including resist thickness. Variations in parameters such as resist thickness and substrate reflectivity tend to create resist images that differ from the intended size. Variations that might be tolerated at 1.25 .mu.m device dimensions become unacceptable at 0.35 .mu.m device dimensions. For example, a 0.1 .mu.m variation in field effect transistor gate width may be acceptable for channel lengths of 1.25 .mu.m but will be unacceptable for channel lengths of 0.35 .mu.m.
In attempts to insure accurate pattern transfer from mask to substrate, multilayer resists have been developed. See, for example, U.S. Pat. No. 4,244,799 issued on Jan. 13, 1981 for a description of a trilayer structure. See, for example, U.S. Pat. Nos. 4,521,274 issued on Jun. 4, 1985 to Reichmanis et al. and 4,657,629 issued on Apr. 14, 1987 to Bigelow for descriptions of bilayer structures. Of course, accurate exposure of the resist is not sufficient for accurate pattern transfer into the planarizing layer; the planarizing layer must also be accurately etched. This is not a trivial endeavor for small features; the resist must be etched in the vertical direction but not in the horizontal direction. Etching in the horizontal direction will, of course, inevitably lead to a change in feature size.
Additional methods for accurate etching of the planarizing layer under the resist are desirable.